Dog red blood cells differ significantly from human red blood cells both in their cation content and in the methods used to maintain this content. It is proposed that this difference may help us to understand the basic mode of ion transport across cell membranes. Specifically we are attempting to characterize the mechanism in dog red cells which cause the transport of cations to be very dependent on cell volume. In addition a basic inhomogeneity has been found in the population of dog red blood cells whereby one fraction of the cells swell in a high potassium buffer and the other fraction shrinks. It has been suggested that this phenomenon may also aid in understanding cation transport in dog red blood cells. Experiments are being carried out to characterize the fluxes of sodium and potassium while regulating the cation concentrations and relative cell volumes. Similar work is also being undertaken to identify the metabolic processes involved in ion transport in dog red cells.